The 20th Greek letter is Upsilon, which has seven letters. It is a vowel in the Greek alphabet and is pronounced as 'oo-psi-lon'. It is also known as Ypsilon or Ypsilon. In mathematics, it is used to represent the Y-axis in the Cartesian coordinate system. In physics, it is used to represent the angular frequency of a wave. In chemistry, it is used to represent the element Yttrium. In astronomy, it is used to represent the Y-axis of the galactic coordinate system.